text: Living organisms produce retinal by irreversible oxidative cleavage of carotenoids.
  For example:
  :beta-carotene + O → 2 retinal,
  catalyzed by a beta-carotene 15,15-monooxygenase or a beta-carotene 15,15-dioxygenase.
  Just as carotenoids are the precursors of retinal, retinal is the precursor of the other forms of vitamin A.  Retinal is interconvertible with retinol, the transport and storage form of vitamin A:
  :retinal + NADPH + H  retinol + NADP
  :retinol + NAD  retinal + NADH + H,
  catalyzed by retinol dehydrogenases (RDHs) and alcohol dehydrogenases (ADHs).
  Retinol is called vitamin A alcohol or, more often, simply vitamin A. Retinal can also be oxidized to retinoic acid:
  :retinal + NAD + HO → retinoic acid + NADH + H (catalyzed by RALDH)
  :retinal + O + HO → retinoic acid + HO (catalyzed by retinal oxidase),
  catalyzed by retinal dehydrogenases also known as retinaldehyde dehydrogenases (RALDHs) as well as retinal oxidases.
  Retinoic acid, sometimes called vitamin A acid, is an important signaling molecule and hormone in vertebrate animals.